About 200 temperature sensitive mutants of the small free living nematode, C. elegans have been isolated. These mutants are blocked in specific stages of growth, gonadogenesis, early or late embryogenesis. A detailed morphological study is undertaken to elucidate the specific steps in normal development and to characterize the phenotypes of the mutants. Temperature shift experiments are used to determine the time during development that a mutant function is required. Genetic studies are employed to determine whether the mutations are inherited maternally, to establish the complementation groups, and to determine the map positions. The results are assembled into a developmental map to describe when and how often a function is required, the duration of its requirement, and the development pathway in which it participates. This map is further refined through biochemical studies to reflect the changes in the genome and its expression during normal and mutant development.